FFC did mean that The Godfather Part III IS the epilogue to the Godfather story, but he also stated that the film's original title was to be THE DEATH OF MICHAEL CORLEONE.

I believe a prequel can be done, focusing on the years before PART I and after Don Ciccio's death in PART II flashbacks, showing Don Vito's steady rise to power and also, showcase the above points mtoppi mentioned.

Then a sequel could happen, shocasing the family's fall.

The prequel would be called RISE OF AN EMPIRE.

Followed by THE GODFATHER TRILOGY (1972 and 1974 and 1990)

And then followed by FALL OF AN EMPIRE.

But then again I am against the flashback trick that was used in PART II. It would be too obvious of a rip-off and may not serve the story well, if not written exceptionally well and deeply thought off. So a GODFATHER PART IV is very tricky...